This invention relates to a drain sheet material to be used for draining excessive pore water from soil.
In a known drain sheet material, a plurality of grooves or channels are formed in the lengthwise direction of a sheet material made of water-permeable paper or non-woven fabric for draining pore water from soil. However, in such a structure of the drain sheet material, when the sheet material in the wet condition is compressed by high earth pressure, the grooves in the sheet material are deformed with the result that the drainage of the pore water will be greatly deteriorated. On the other hand, when the sheet material is made of hard material to prevent the grooves therein from being deformed by the earth pressure, the sheet material itself will be broken easily and the water-permeability thereof will be less. Thus, it has been difficult to provide a drain sheet material in which the drain function of the grooves is maintained without deteriorating the water-permeability of the sheet material itself even when it is used under high earth pressure.